The present invention relates to a method for starting a multi-cylinder internal combustion engine, in particular of a motor vehicle, in the forward direction, wherein the position of a piston in a cylinder of the engine is ascertained, and fuel is injected into a combustion chamber of the particular cylinder whose piston is in a working phase.
The invention also relates to a multi-cylinder internal combustion engine, in particular of a motor vehicle. The engine includes a detector for ascertaining the position of a piston in a cylinder of the engine and a fuel metering system for injecting fuel into a combustion chamber of the particular cylinder whose piston is in a working phase. Finally, the present invention also relates to a control unit for a multi-cylinder internal combustion engine of this kind, in particular of a motor vehicle.
A method for starting a multi-cylinder internal combustion engine of the type defined at the outset is known for instance from German Patent Disclosure DE 31 17 144 A1. The method described there operates without an electric-motor starter. When the engine is at a stop, a quantity of fuel required for combustion is injected into the combustion chamber of one or more cylinders (starting cylinders), whose pistons are in the working phase, and is ignited. After that, fuel is injected into the combustion chamber of the cylinder or cylinders whose pistons are executing the next working stroke, and is ignited as soon as the applicable pistons have reached the working position. In this way, the engine can be embodied without an electric starter and the associated components required by such a starter. Moreover, a battery of the engine can be made smaller, since it no longer has to furnish energy for the starter and the other electrical components.
In the known method for starting an internal combustion engine, at one piston position of the starting cylinder near top dead center, only a relatively small quantity of air is contained in the combustion chamber of the starting cylinder. The resultant combustion energy from the combustion of the fuel injected into the combustion chamber can under some circumstances, because of the small air mass, may furnish too little starting energy, preventing the machine from being started. Moreover, the spacing between an injection valve, by way of which the fuel is injected into the combustion chamber, and the piston may be too slight, so that the fuel injected, as a consequence of penetration, changes virtually completely into a piston wall film that is hardly capable of evaporating.
German Patent Disclosure DE 197 43 492 A1 can also be referred to as further prior art; once again, it discloses a method for starting an internal combustion engine without an electric starter.
The present invention has the object of reliably starting a multi-cylinder internal combustion engine without an electric starter, regardless of the position of the pistons in the cylinders before the starting process.
For attaining this object, the invention, based on the method of the type defined at the outset, proposes that the engine is first moved in the reverse direction, by the injection of fuel into a combustion chamber of at least one cylinder whose piston isxe2x80x94viewed in the forward directionxe2x80x94in a compression phase, and the fuel compressed in the combustion chamber of the at least one cylinder is ignited, and the rotary motion in the reverse direction comes to a stop before the bottom dead center of the pistons of the at least one cylinder is reached, and that the engine is then started in the forward direction.
According to the invention, before the starter-free starting, the engine is accordingly first moved in reverse far enough that the pistons in the starting cylinder are in an optimal starting position. Since for starting the engine in the forward direction fuel is injected into the combustion chamber of a cylinder whose piston is in a working phase, the optimal starting position of the pistonsxe2x80x94viewed in the forward directionxe2x80x94is immediately after top dead center. Because of this position of the pistons, combustion of the fuel injected into the combustion chamber of the starting cylinder can generate especially high combustion energy and thus also especially high starting energy.
Moreover, according to the invention, during the reverse motion of the engine, a relatively large air mass is aspirated into the combustion chamber of the particular cylinder whichxe2x80x94viewed in the forward directionxe2x80x94is in the working phase. It can therefore be assured that the combustion energy, resulting from the combustion of the fuel injected into the combustion chamber of the starting cylinder, furnishes adequately high starting energy to enable reliable starting of the engine.
Finally, as a result of the reverse motion of the engine before the starting in the forward direction, the piston of the starting cylinder is moved away by the injection valve, so that when the fuel is injected into the combustion chamber of the starting cylinder, only very slight penetration, if any, occurs, and the injected fuel changes over virtually completely into an easily ignitable fuel-air mixture in the form of a fuel cloud.
In an advantageous refinement of the present invention, it is proposed that inlet and/or outlet valves of the at least one cylinder, whose piston is locatedxe2x80x94viewed in the forward directionxe2x80x94before its top dead center is put, before the starting process, into a position corresponding to the compression phase. To enable putting the valves into a predeterminable position, regardless of the engine, a camshaft-free control of the inlet and/or outlet valves is needed. Thus each inlet valve and outlet valve can be triggered separately from the other valves and independently of the position of the camshaft. For camshaft-free control, the inlet and/or outlet valves are equipped either individually or in groups of several jointly with an actuator device. The actuator device may function hydraulically, piezoelectrically, electromagnetically, or in some other way. From the prior art, many camshaft-free controls for inlet and outlet valves are known that can be used in conjunction with the method of the present invention. In accordance with the refinement, the valves can be opened and closed independently andxe2x80x94if the freedom of valve motion allows itxe2x80x94freely. In this way, it is successfully possible before or during the starting process to change from an aspiration phase to a working phase and vice versa. It is correspondingly also possible to change from a compression phase to an expulsion phase and vice versa.
In a preferred embodiment of the present invention, it is proposed that the inlet and/or outlet valves of two cylinders, whose pistons are locatedxe2x80x94viewed in the forward directionxe2x80x94before their top dead center are brought, before the starting process, into a position corresponding to the compression phase. Hence the engine is first put in a reverse direction, by injecting fuel into the combustion chambers of two cylinders whose pistons arexe2x80x94viewed in the forward directionxe2x80x94in a compression phase. Then, the fuel compressed in the combustion chamber of the two cylinders is ignited. As a result of the double combustion, sufficiently high combustion energy and thus a sufficiently starting energy are generated to overcome any static frictional or frictional and compression resistances of the engine, and initially to put the engine in a reverse motion reliably.
In another preferred embodiment of the present invention, it is proposed that during the rotary motion of the engine in the reverse direction, the inlet and/or outlet valves of a cylinder, whose piston is locatedxe2x80x94viewed in the forward directionxe2x80x94in an aspiration phase, are actuated in a targeted way such that the rotary motion of the engine in the reverse direction comes to a stop before bottom dead center of the pistons of the at least one cylinder is reached. By closing the inlet valves and outlet valves of a cylinder whose piston is in an aspiration phase, at the onset of the method of the invention or during the reverse motion of the engine, a pressure can be built up during the reverse motion in the combustion chamber by which the reverse motion is braked. By purposeful opening of the inlet and/or outlet valves, the level of the pressure building up in the combustion chamber during the reverse motion can be controlled, so that the rotary motion of the engine in the reverse direction comes to a stop precisely before bottom dead center of the pistons of the at least one cylinder is reached.
Advantageously, the inlet and outlet valves of the cylinder whose piston is locatedxe2x80x94viewed in the forward directionxe2x80x94in an aspiration phase, are closed during the rotary motion of the engine in the reverse direction.
Preferably, the inlet and outlet valves of the cylinder whose piston is locatedxe2x80x94viewed in the forward directionxe2x80x94in an aspiration phase, are kept closed for a predeterminable period of time after the reversal of the direction of rotation of the engine. As a result, the compression energy stored in the combustion chamber can be used to accelerate the crankshaft in the forward direction.
In another advantageous refinement of the present invention, it is proposed that during the rotary motion of the engine in the reverse direction, fuel is injected into a combustion chamber of a further cylinder, whose piston is locatedxe2x80x94viewed in the forward directionxe2x80x94in a working phase, and the fuel compressed in the combustion chamber of the at least one cylinder is ignited beforexe2x80x94viewed in the reverse directionxe2x80x94the top dead center is reached. During the reverse motion of the engine, the injected fuel is compressed in the combustion chamber and finally ignited just before top dead center is reached. As a result of the compression of the fuel, the reverse motionxe2x80x94if it has not yet occurredxe2x80x94is braked to a standstill. Then by the ignition of the fuel, the engine is set into an opposed forward motion. This initiates the starter-free starting process in the forward direction.
In still another preferred embodiment of the present invention, then in the further course of the starting process, fuel is injected into a combustion chamber of a cylinder, whose piston is locatedxe2x80x94viewed in the forward directionxe2x80x94in an aspiration phase or a compression phase, and the fuel compressed in the combustion chamber of the at least one cylinder is ignited. The onset of injection into the combustion chamber of the further cylinder occurs for instance in the aspiration phase of the piston and takes place at an injection pressure which is built up by a prefeed pump, driven independently of the engine, of the fuel metering system. The prefeed pump is embodied for instance as an electric fuel pump driven independently of the engine. A prefeed pump, in a common rail fuel metering system, for instance, serves to pump fuel out of a fuel reservoir into a low-pressure region of the fuel metering system. However, the onset of injection canxe2x80x94if the injection pressure is high enoughxe2x80x94also be shifted into the continuing compression phase until just before top dead center is reached. This kind of high injection pressure can be generated for instance by a high-pressure pump, operated independently of the engine, of the fuel metering system. In a common rail fuel metering system, for instance, the high-pressure pump pumps fuel out of the low-pressure region of the fuel metering system at high pressure into a high-pressure reservoir. From the high-pressure reservoir, injection valves branch off, by way of which fuel is injected out of the high-pressure reservoir into the combustion chambers of the cylinders. The high-pressure pump can be driven electrically, for instance. As a result of the combustion of the fuel injected into the combustion chamber of the cylinder, the rotary motion of the crankshaft in the forward direction is accelerated still further.
A proposed embodiment also covers the case where fuel, during the reverse motion of the engine, is injected into a combustion chamber of a cylinder whose piston isxe2x80x94viewed in the forward directionxe2x80x94in an expulsion phase. This is equivalent during the reverse motion of the engine to an aspiration phase. The fuel injected into this cylinder can then, during the forward motion of the engine, be ignited in the compression phase, preferably toward the end of the compression phase. It is understood that in this case as well, the injection onset can be shifted into the continuing compression phasexe2x80x94during the forward motion of the engine.
From the method of the invention, additional degrees of freedom are obtained in the starting process, and these can be utilized, among other purposes, for initiating a second attempt at starting after an unsuccessful first ignition. The first ignition may for instance be unsuccessful if the engine is not moving in the reverse direction, or if the first compression resistance could not be overcome. In a preferred embodiment of the present invention, it is proposed that after an first ignition of the fuel injected into the at least one cylinder has failed to succeed, the method is performed again, with inverted phases of the individual cylinders. That is, the method of the invention is accordinglyxe2x80x94specifically, with inverted phases of the individual cylindersxe2x80x94performed. This means that by suitable actuation of the inlet and/or outlet valves, the cylinders thatxe2x80x94viewed in the forward directionxe2x80x94were in a compression phase during the first attempt at starting are in an expulsion phase during the second attempt at starting, and vice versa. Moreover, the cylinders that during the first attempt at starting were in a working phase are shifted to an aspiration phase in the second attempt at starting, and vice versa. In the second attempt at starting, the injection of fuel into the combustion chambers and the ignition of the compressed fuel occur as described above.
To reduce the compression resistance during the starting process of the invention, in a preferred embodiment of the present invention it is proposed that during the starting process in a compression phase of a cylinder of the engine, the corresponding inlet valve of the cylinder is closed late. As a result, every compression phase that has been executed can advantageously be shortened by delayed closure of the corresponding inlet valvesxe2x80x94which are opened during the aspiration phase that takes place before the compression phase. In this way, the crankshaft of the engine, because of the combustion at the onset of the starting process, can be much more easily put into a rotary motion in the forward direction, and the engine can be started correspondingly more easily.
Advantageously, the fuel compressed in a combustion chamber of a cylinder is ignited just before the top dead center of the piston of the applicable cylinder is reached, toward the end of the compression phase.
The realization of the method of the invention in the form of a control element, which is provided for a control unit of an engine, particularly of a motor vehicle, is of particular importance. In the control element, a program is stored in memory which can be run on a computing device, in particular a microprocessor, and which is suitable for performing the method of the invention. In this case, the invention is accordingly realized by means of a program stored in memory in the control element, so that this control element provided with the program represents the invention in the same way as does the method for whose performance the program is suited. As the control element, an electric storage medium can be used in particular, such as a read-only memory or a flash memory.
As a further way of attaining the object of the present invention, based on the multi-cylinder internal combustion engine of the type defined at the outset, it is proposed that the engine has means for performing the method of one of claims 1-11.
In an advantageous refinement of the present invention, it is proposed that the engine has a camshaft-free control of the inlet and/or outlet valves of the combustion chambers.
In a preferred embodiment of the present invention, it is proposed that the fuel metering system has a high-pressure pump, driven independently of the engine, for building up a fuel injection pressure.
As still another way of attaining the object of the present invention, it is proposed, based on the control unit of the type defined above, that the control unit has means for performing the method of one of claims 1-11. Accordingly, for starting an engine, the control unit not only carries out triggering of components of the engine that are involved in the starting process of the invention, in particular components of the fuel metering system and of the ignition. The control unit receives the command to start the engine from the actuation of an ignition key or starter button, for instance.